-@c Copyright (C) 1996, 1997, 1999, 2000 Free Software Foundation, Inc.
+@c Copyright (C) 1996, 1997, 1999, 2000, 2001,
+@c 2002, 2003, 2004, 2005, 2008 Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.
@ignore
@c man begin COPYRIGHT
-Copyright @copyright{} 1996, 1997, 1999, 2000 Free Software Foundation, Inc.
-
-Permission is granted to make and distribute verbatim copies of this
-manual provided the copyright notice and this permission notice are
-preserved on all copies.
-
-Permission is granted to copy and distribute modified versions of this
-manual under the conditions for verbatim copying, provided also that the
-entire resulting derived work is distributed under the terms of a
-permission notice identical to this one.
-
-Permission is granted to copy and distribute translations of this manual
-into another language, under the above conditions for modified versions,
-except that this permission notice may be included in translations
-approved by the Free Software Foundation instead of in the original
-English.
+Copyright @copyright{} 1996, 1997, 1999, 2000, 2001, 2002, 2003, 2004, 2005,
+2008 Free Software Foundation, Inc.
+
+Permission is granted to copy, distribute and/or modify this document
+under the terms of the GNU Free Documentation License, Version 1.2 or
+any later version published by the Free Software Foundation; with the
+Invariant Sections being ``GNU General Public License'' and ``Funding
+Free Software'', the Front-Cover texts being (a) (see below), and with
+the Back-Cover Texts being (b) (see below). A copy of the license is
+included in the gfdl(7) man page.
+
+(a) The FSF's Front-Cover Text is:
+
+ A GNU Manual
+
+(b) The FSF's Back-Cover Text is:
+
+ You have freedom to copy and modify this GNU Manual, like GNU
+ software. Copies published by the Free Software Foundation raise
+ funds for GNU development.
@c man end
@c Set file name and title for the man page.
@setfilename gcov
@end ignore
@node Gcov
-@chapter @code{gcov}: a Test Coverage Program
+@chapter @command{gcov}---a Test Coverage Program
-@code{gcov} is a tool you can use in conjunction with @sc{gnu} CC to
+@command{gcov} is a tool you can use in conjunction with GCC to
test code coverage in your programs.
-This chapter describes version 1.5 of @code{gcov}.
-
@menu
-* Gcov Intro:: Introduction to gcov.
-* Invoking Gcov:: How to use gcov.
+* Gcov Intro:: Introduction to gcov.
+* Invoking Gcov:: How to use gcov.
* Gcov and Optimization:: Using gcov with GCC optimization.
* Gcov Data Files:: The files used by gcov.
+* Cross-profiling:: Data file relocation.
@end menu
@node Gcov Intro
-@section Introduction to @code{gcov}
+@section Introduction to @command{gcov}
@c man begin DESCRIPTION
-@code{gcov} is a test coverage program. Use it in concert with @sc{gnu}
-CC to analyze your programs to help create more efficient, faster
-running code. You can use @code{gcov} as a profiling tool to help
-discover where your optimization efforts will best affect your code. You
-can also use @code{gcov} along with the other profiling tool,
-@code{gprof}, to assess which parts of your code use the greatest amount
-of computing time.
+@command{gcov} is a test coverage program. Use it in concert with GCC
+to analyze your programs to help create more efficient, faster running
+code and to discover untested parts of your program. You can use
+@command{gcov} as a profiling tool to help discover where your
+optimization efforts will best affect your code. You can also use
+@command{gcov} along with the other profiling tool, @command{gprof}, to
+assess which parts of your code use the greatest amount of computing
+time.
Profiling tools help you analyze your code's performance. Using a
-profiler such as @code{gcov} or @code{gprof}, you can find out some
+profiler such as @command{gcov} or @command{gprof}, you can find out some
basic performance statistics, such as:
@itemize @bullet
Once you know these things about how your code works when compiled, you
can look at each module to see which modules should be optimized.
-@code{gcov} helps you determine where to work on optimization.
+@command{gcov} helps you determine where to work on optimization.
Software developers also use coverage testing in concert with
testsuites, to make sure software is actually good enough for a release.
final product.
You should compile your code without optimization if you plan to use
-@code{gcov} because the optimization, by combining some lines of code
+@command{gcov} because the optimization, by combining some lines of code
into one function, may not give you as much information as you need to
look for `hot spots' where the code is using a great deal of computer
-time. Likewise, because @code{gcov} accumulates statistics by line (at
+time. Likewise, because @command{gcov} accumulates statistics by line (at
the lowest resolution), it works best with a programming style that
places only one statement on each line. If you use complicated macros
that expand to loops or to other control structures, the statistics are
appears. If your complex macros behave like functions, you can replace
them with inline functions to solve this problem.
-@code{gcov} creates a logfile called @file{@var{sourcefile}.gcov} which
+@command{gcov} creates a logfile called @file{@var{sourcefile}.gcov} which
indicates how many times each line of a source file @file{@var{sourcefile}.c}
-has executed. You can use these logfiles along with @code{gprof} to aid
-in fine-tuning the performance of your programs. @code{gprof} gives
+has executed. You can use these logfiles along with @command{gprof} to aid
+in fine-tuning the performance of your programs. @command{gprof} gives
timing information you can use along with the information you get from
-@code{gcov}.
+@command{gcov}.
-@code{gcov} works only on code compiled with @sc{gnu} CC. It is not
+@command{gcov} works only on code compiled with GCC@. It is not
compatible with any other profiling or test coverage mechanism.
@c man end
@node Invoking Gcov
-@section Invoking gcov
+@section Invoking @command{gcov}
@smallexample
-gcov [-b] [-c] [-v] [-n] [-l] [-f] [-o directory] @var{sourcefile}
+gcov @r{[}@var{options}@r{]} @var{sourcefiles}
@end smallexample
+@command{gcov} accepts the following options:
+
@ignore
@c man begin SYNOPSIS
-gcov [@samp{-b}] [@samp{-c}] [@samp{-v}] [@samp{-n}] [@samp{-l}] [@samp{-f}] [@samp{-o} @var{directory}] @var{sourcefile}
+gcov [@option{-v}|@option{--version}] [@option{-h}|@option{--help}]
+ [@option{-a}|@option{--all-blocks}]
+ [@option{-b}|@option{--branch-probabilities}]
+ [@option{-c}|@option{--branch-counts}]
+ [@option{-n}|@option{--no-output}]
+ [@option{-l}|@option{--long-file-names}]
+ [@option{-p}|@option{--preserve-paths}]
+ [@option{-f}|@option{--function-summaries}]
+ [@option{-o}|@option{--object-directory} @var{directory|file}] @var{sourcefiles}
+ [@option{-u}|@option{--unconditional-branches}]
@c man end
@c man begin SEEALSO
-gcc(1) and the Info entry for @file{gcc}.
+gpl(7), gfdl(7), fsf-funding(7), gcc(1) and the Info entry for @file{gcc}.
@c man end
@end ignore
@c man begin OPTIONS
-@table @code
+@table @gcctabopt
+@item -h
+@itemx --help
+Display help about using @command{gcov} (on the standard output), and
+exit without doing any further processing.
+
+@item -v
+@itemx --version
+Display the @command{gcov} version number (on the standard output),
+and exit without doing any further processing.
+
+@item -a
+@itemx --all-blocks
+Write individual execution counts for every basic block. Normally gcov
+outputs execution counts only for the main blocks of a line. With this
+option you can determine if blocks within a single line are not being
+executed.
+
@item -b
+@itemx --branch-probabilities
Write branch frequencies to the output file, and write branch summary
info to the standard output. This option allows you to see how often
-each branch in your program was taken.
+each branch in your program was taken. Unconditional branches will not
+be shown, unless the @option{-u} option is given.
@item -c
+@itemx --branch-counts
Write branch frequencies as the number of branches taken, rather than
the percentage of branches taken.
-@item -v
-Display the @code{gcov} version number (on the standard error stream).
-
@item -n
-Do not create the @code{gcov} output file.
+@itemx --no-output
+Do not create the @command{gcov} output file.
@item -l
+@itemx --long-file-names
Create long file names for included source files. For example, if the
-header file @samp{x.h} contains code, and was included in the file
-@samp{a.c}, then running @code{gcov} on the file @samp{a.c} will produce
-an output file called @samp{a.c.x.h.gcov} instead of @samp{x.h.gcov}.
-This can be useful if @samp{x.h} is included in multiple source files.
+header file @file{x.h} contains code, and was included in the file
+@file{a.c}, then running @command{gcov} on the file @file{a.c} will produce
+an output file called @file{a.c##x.h.gcov} instead of @file{x.h.gcov}.
+This can be useful if @file{x.h} is included in multiple source
+files. If you use the @samp{-p} option, both the including and
+included file names will be complete path names.
+
+@item -p
+@itemx --preserve-paths
+Preserve complete path information in the names of generated
+@file{.gcov} files. Without this option, just the filename component is
+used. With this option, all directories are used, with @samp{/} characters
+translated to @samp{#} characters, @file{.} directory components
+removed and @file{..}
+components renamed to @samp{^}. This is useful if sourcefiles are in several
+different directories. It also affects the @samp{-l} option.
@item -f
+@itemx --function-summaries
Output summaries for each function in addition to the file level summary.
-@item -o
-The directory where the object files live. Gcov will search for @code{.bb},
-@code{.bbg}, and @code{.da} files in this directory.
+@item -o @var{directory|file}
+@itemx --object-directory @var{directory}
+@itemx --object-file @var{file}
+Specify either the directory containing the gcov data files, or the
+object path name. The @file{.gcno}, and
+@file{.gcda} data files are searched for using this option. If a directory
+is specified, the data files are in that directory and named after the
+source file name, without its extension. If a file is specified here,
+the data files are named after that file, without its extension. If this
+option is not supplied, it defaults to the current directory.
+
+@item -u
+@itemx --unconditional-branches
+When branch probabilities are given, include those of unconditional branches.
+Unconditional branches are normally not interesting.
+
@end table
-@need 3000
-When using @code{gcov}, you must first compile your program with two
-special @sc{gnu} CC options: @samp{-fprofile-arcs -ftest-coverage}.
+@command{gcov} should be run with the current directory the same as that
+when you invoked the compiler. Otherwise it will not be able to locate
+the source files. @command{gcov} produces files called
+@file{@var{mangledname}.gcov} in the current directory. These contain
+the coverage information of the source file they correspond to.
+One @file{.gcov} file is produced for each source file containing code,
+which was compiled to produce the data files. The @var{mangledname} part
+of the output file name is usually simply the source file name, but can
+be something more complicated if the @samp{-l} or @samp{-p} options are
+given. Refer to those options for details.
+
+The @file{.gcov} files contain the @samp{:} separated fields along with
+program source code. The format is
+
+@smallexample
+@var{execution_count}:@var{line_number}:@var{source line text}
+@end smallexample
+
+Additional block information may succeed each line, when requested by
+command line option. The @var{execution_count} is @samp{-} for lines
+containing no code and @samp{#####} for lines which were never executed.
+Some lines of information at the start have @var{line_number} of zero.
+
+The preamble lines are of the form
+
+@smallexample
+-:0:@var{tag}:@var{value}
+@end smallexample
+
+The ordering and number of these preamble lines will be augmented as
+@command{gcov} development progresses --- do not rely on them remaining
+unchanged. Use @var{tag} to locate a particular preamble line.
+
+The additional block information is of the form
+
+@smallexample
+@var{tag} @var{information}
+@end smallexample
+
+The @var{information} is human readable, but designed to be simple
+enough for machine parsing too.
+
+When printing percentages, 0% and 100% are only printed when the values
+are @emph{exactly} 0% and 100% respectively. Other values which would
+conventionally be rounded to 0% or 100% are instead printed as the
+nearest non-boundary value.
+
+When using @command{gcov}, you must first compile your program with two
+special GCC options: @samp{-fprofile-arcs -ftest-coverage}.
This tells the compiler to generate additional information needed by
gcov (basically a flow graph of the program) and also includes
additional code in the object files for generating the extra profiling
information needed by gcov. These additional files are placed in the
-directory where the source code is located.
+directory where the object file is located.
Running the program will cause profile output to be generated. For each
-source file compiled with -fprofile-arcs, an accompanying @code{.da}
-file will be placed in the source directory.
+source file compiled with @option{-fprofile-arcs}, an accompanying
+@file{.gcda} file will be placed in the object file directory.
-Running @code{gcov} with your program's source file names as arguments
+Running @command{gcov} with your program's source file names as arguments
will now produce a listing of the code along with frequency of execution
-for each line. For example, if your program is called @samp{tmp.c}, this
-is what you see when you use the basic @code{gcov} facility:
+for each line. For example, if your program is called @file{tmp.c}, this
+is what you see when you use the basic @command{gcov} facility:
@smallexample
$ gcc -fprofile-arcs -ftest-coverage tmp.c
$ a.out
$ gcov tmp.c
- 87.50% of 8 source lines executed in file tmp.c
+90.00% of 10 source lines executed in file tmp.c
Creating tmp.c.gcov.
@end smallexample
-The file @file{tmp.c.gcov} contains output from @code{gcov}.
+The file @file{tmp.c.gcov} contains output from @command{gcov}.
Here is a sample:
@smallexample
- main()
- @{
- 1 int i, total;
-
- 1 total = 0;
+ -: 0:Source:tmp.c
+ -: 0:Graph:tmp.gcno
+ -: 0:Data:tmp.gcda
+ -: 0:Runs:1
+ -: 0:Programs:1
+ -: 1:#include <stdio.h>
+ -: 2:
+ -: 3:int main (void)
+ 1: 4:@{
+ 1: 5: int i, total;
+ -: 6:
+ 1: 7: total = 0;
+ -: 8:
+ 11: 9: for (i = 0; i < 10; i++)
+ 10: 10: total += i;
+ -: 11:
+ 1: 12: if (total != 45)
+ #####: 13: printf ("Failure\n");
+ -: 14: else
+ 1: 15: printf ("Success\n");
+ 1: 16: return 0;
+ -: 17:@}
+@end smallexample
- 11 for (i = 0; i < 10; i++)
- 10 total += i;
+When you use the @option{-a} option, you will get individual block
+counts, and the output looks like this:
- 1 if (total != 45)
- ###### printf ("Failure\n");
- else
- 1 printf ("Success\n");
- 1 @}
+@smallexample
+ -: 0:Source:tmp.c
+ -: 0:Graph:tmp.gcno
+ -: 0:Data:tmp.gcda
+ -: 0:Runs:1
+ -: 0:Programs:1
+ -: 1:#include <stdio.h>
+ -: 2:
+ -: 3:int main (void)
+ 1: 4:@{
+ 1: 4-block 0
+ 1: 5: int i, total;
+ -: 6:
+ 1: 7: total = 0;
+ -: 8:
+ 11: 9: for (i = 0; i < 10; i++)
+ 11: 9-block 0
+ 10: 10: total += i;
+ 10: 10-block 0
+ -: 11:
+ 1: 12: if (total != 45)
+ 1: 12-block 0
+ #####: 13: printf ("Failure\n");
+ $$$$$: 13-block 0
+ -: 14: else
+ 1: 15: printf ("Success\n");
+ 1: 15-block 0
+ 1: 16: return 0;
+ 1: 16-block 0
+ -: 17:@}
@end smallexample
+In this mode, each basic block is only shown on one line -- the last
+line of the block. A multi-line block will only contribute to the
+execution count of that last line, and other lines will not be shown
+to contain code, unless previous blocks end on those lines.
+The total execution count of a line is shown and subsequent lines show
+the execution counts for individual blocks that end on that line. After each
+block, the branch and call counts of the block will be shown, if the
+@option{-b} option is given.
+
+Because of the way GCC instruments calls, a call count can be shown
+after a line with no individual blocks.
+As you can see, line 13 contains a basic block that was not executed.
+
@need 450
-When you use the @samp{-b} option, your output looks like this:
+When you use the @option{-b} option, your output looks like this:
@smallexample
$ gcov -b tmp.c
- 87.50% of 8 source lines executed in file tmp.c
- 80.00% of 5 branches executed in file tmp.c
- 80.00% of 5 branches taken at least once in file tmp.c
- 50.00% of 2 calls executed in file tmp.c
+90.00% of 10 source lines executed in file tmp.c
+80.00% of 5 branches executed in file tmp.c
+80.00% of 5 branches taken at least once in file tmp.c
+50.00% of 2 calls executed in file tmp.c
Creating tmp.c.gcov.
@end smallexample
Here is a sample of a resulting @file{tmp.c.gcov} file:
@smallexample
- main()
- @{
- 1 int i, total;
-
- 1 total = 0;
-
- 11 for (i = 0; i < 10; i++)
-branch 0 taken = 91%
-branch 1 taken = 100%
-branch 2 taken = 100%
- 10 total += i;
-
- 1 if (total != 45)
-branch 0 taken = 100%
- ###### printf ("Failure\n");
-call 0 never executed
-branch 1 never executed
- else
- 1 printf ("Success\n");
-call 0 returns = 100%
- 1 @}
+ -: 0:Source:tmp.c
+ -: 0:Graph:tmp.gcno
+ -: 0:Data:tmp.gcda
+ -: 0:Runs:1
+ -: 0:Programs:1
+ -: 1:#include <stdio.h>
+ -: 2:
+ -: 3:int main (void)
+function main called 1 returned 1 blocks executed 75%
+ 1: 4:@{
+ 1: 5: int i, total;
+ -: 6:
+ 1: 7: total = 0;
+ -: 8:
+ 11: 9: for (i = 0; i < 10; i++)
+branch 0 taken 91% (fallthrough)
+branch 1 taken 9%
+ 10: 10: total += i;
+ -: 11:
+ 1: 12: if (total != 45)
+branch 0 taken 0% (fallthrough)
+branch 1 taken 100%
+ #####: 13: printf ("Failure\n");
+call 0 never executed
+ -: 14: else
+ 1: 15: printf ("Success\n");
+call 0 called 1 returned 100%
+ 1: 16: return 0;
+ -: 17:@}
@end smallexample
+For each function, a line is printed showing how many times the function
+is called, how many times it returns and what percentage of the
+function's blocks were executed.
+
For each basic block, a line is printed after the last line of the basic
block describing the branch or call that ends the basic block. There can
be multiple branches and calls listed for a single source line if there
For a call, if it was executed at least once, then a percentage
indicating the number of times the call returned divided by the number
of times the call was executed will be printed. This will usually be
-100%, but may be less for functions call @code{exit} or @code{longjmp},
+100%, but may be less for functions that call @code{exit} or @code{longjmp},
and thus may not return every time they are called.
The execution counts are cumulative. If the example program were
-executed again without removing the @code{.da} file, the count for the
+executed again without removing the @file{.gcda} file, the count for the
number of times each line in the source was executed would be added to
the results of the previous run(s). This is potentially useful in
several ways. For example, it could be used to accumulate data over a
provide more accurate long-term information over a large number of
program runs.
-The data in the @code{.da} files is saved immediately before the program
-exits. For each source file compiled with -fprofile-arcs, the profiling
-code first attempts to read in an existing @code{.da} file; if the file
-doesn't match the executable (differing number of basic block counts) it
-will ignore the contents of the file. It then adds in the new execution
-counts and finally writes the data to the file.
+The data in the @file{.gcda} files is saved immediately before the program
+exits. For each source file compiled with @option{-fprofile-arcs}, the
+profiling code first attempts to read in an existing @file{.gcda} file; if
+the file doesn't match the executable (differing number of basic block
+counts) it will ignore the contents of the file. It then adds in the
+new execution counts and finally writes the data to the file.
@node Gcov and Optimization
-@section Using @code{gcov} with GCC Optimization
+@section Using @command{gcov} with GCC Optimization
-If you plan to use @code{gcov} to help optimize your code, you must
-first compile your program with two special @sc{gnu} CC options:
+If you plan to use @command{gcov} to help optimize your code, you must
+first compile your program with two special GCC options:
@samp{-fprofile-arcs -ftest-coverage}. Aside from that, you can use any
-other @sc{gnu} CC options; but if you want to prove that every single line
+other GCC options; but if you want to prove that every single line
in your program was executed, you should not compile with optimization
at the same time. On some machines the optimizer can eliminate some
simple code lines by combining them with other lines. For example, code
@noindent
can be compiled into one instruction on some machines. In this case,
-there is no way for @code{gcov} to calculate separate execution counts
+there is no way for @command{gcov} to calculate separate execution counts
for each line because there isn't separate code for each line. Hence
-the @code{gcov} output looks like this if you compiled the program with
+the @command{gcov} output looks like this if you compiled the program with
optimization:
@smallexample
- 100 if (a != b)
- 100 c = 1;
- 100 else
- 100 c = 0;
+ 100: 12:if (a != b)
+ 100: 13: c = 1;
+ 100: 14:else
+ 100: 15: c = 0;
@end smallexample
The output shows that this block of code, combined by optimization,
was only one instruction representing all four of these lines. However,
the output does not indicate how many times the result was 0 and how
many times the result was 1.
+
+Inlineable functions can create unexpected line counts. Line counts are
+shown for the source code of the inlineable function, but what is shown
+depends on where the function is inlined, or if it is not inlined at all.
+
+If the function is not inlined, the compiler must emit an out of line
+copy of the function, in any object file that needs it. If
+@file{fileA.o} and @file{fileB.o} both contain out of line bodies of a
+particular inlineable function, they will also both contain coverage
+counts for that function. When @file{fileA.o} and @file{fileB.o} are
+linked together, the linker will, on many systems, select one of those
+out of line bodies for all calls to that function, and remove or ignore
+the other. Unfortunately, it will not remove the coverage counters for
+the unused function body. Hence when instrumented, all but one use of
+that function will show zero counts.
+
+If the function is inlined in several places, the block structure in
+each location might not be the same. For instance, a condition might
+now be calculable at compile time in some instances. Because the
+coverage of all the uses of the inline function will be shown for the
+same source lines, the line counts themselves might seem inconsistent.
+
@c man end
@node Gcov Data Files
-@section Brief description of @code{gcov} data files
-
-@code{gcov} uses three files for doing profiling. The names of these
-files are derived from the original @emph{source} file by substituting
-the file suffix with either @code{.bb}, @code{.bbg}, or @code{.da}. All
-of these files are placed in the same directory as the source file, and
-contain data stored in a platform-independent method.
-
-The @code{.bb} and @code{.bbg} files are generated when the source file
-is compiled with the @sc{gnu} CC @samp{-ftest-coverage} option. The
-@code{.bb} file contains a list of source files (including headers),
-functions within those files, and line numbers corresponding to each
-basic block in the source file.
-
-The @code{.bb} file format consists of several lists of 4-byte integers
-which correspond to the line numbers of each basic block in the
-file. Each list is terminated by a line number of 0. A line number of -1
-is used to designate that the source file name (padded to a 4-byte
-boundary and followed by another -1) follows. In addition, a line number
-of -2 is used to designate that the name of a function (also padded to a
-4-byte boundary and followed by a -2) follows.
-
-The @code{.bbg} file is used to reconstruct the program flow graph for
-the source file. It contains a list of the program flow arcs (possible
-branches taken from one basic block to another) for each function which,
-in combination with the @code{.bb} file, enables gcov to reconstruct the
-program flow.
-
-In the @code{.bbg} file, the format is:
-@smallexample
- number of basic blocks for function #0 (4-byte number)
- total number of arcs for function #0 (4-byte number)
- count of arcs in basic block #0 (4-byte number)
- destination basic block of arc #0 (4-byte number)
- flag bits (4-byte number)
- destination basic block of arc #1 (4-byte number)
- flag bits (4-byte number)
- ...
- destination basic block of arc #N (4-byte number)
- flag bits (4-byte number)
- count of arcs in basic block #1 (4-byte number)
- destination basic block of arc #0 (4-byte number)
- flag bits (4-byte number)
- ...
-@end smallexample
+@section Brief description of @command{gcov} data files
+
+@command{gcov} uses two files for profiling. The names of these files
+are derived from the original @emph{object} file by substituting the
+file suffix with either @file{.gcno}, or @file{.gcda}. All of these files
+are placed in the same directory as the object file, and contain data
+stored in a platform-independent format.
+
+The @file{.gcno} file is generated when the source file is compiled with
+the GCC @option{-ftest-coverage} option. It contains information to
+reconstruct the basic block graphs and assign source line numbers to
+blocks.
+
+The @file{.gcda} file is generated when a program containing object files
+built with the GCC @option{-fprofile-arcs} option is executed. A
+separate @file{.gcda} file is created for each object file compiled with
+this option. It contains arc transition counts, and some summary
+information.
+
+The full details of the file format is specified in @file{gcov-io.h},
+and functions provided in that header file should be used to access the
+coverage files.
+
+@node Cross-profiling
+@section Data file relocation to support cross-profiling
+
+Running the program will cause profile output to be generated. For each
+source file compiled with @option{-fprofile-arcs}, an accompanying @file{.gcda}
+file will be placed in the object file directory. That implicitly requires
+running the program on the same system as it was built or having the same
+absolute directory structure on the target system. The program will try
+to create the needed directory structure, if it is not already present.
+
+To support cross-profiling, a program compiled with @option{-fprofile-arcs}
+can relocate the data files based on two environment variables:
-A -1 (stored as a 4-byte number) is used to separate each function's
-list of basic blocks, and to verify that the file has been read
-correctly.
-
-The @code{.da} file is generated when a program containing object files
-built with the @sc{gnu} CC @samp{-fprofile-arcs} option is executed. A
-separate @code{.da} file is created for each source file compiled with
-this option, and the name of the @code{.da} file is stored as an
-absolute pathname in the resulting object file. This path name is
-derived from the source file name by substituting a @code{.da} suffix.
-
-The format of the @code{.da} file is fairly simple. The first 8-byte
-number is the number of counts in the file, followed by the counts
-(stored as 8-byte numbers). Each count corresponds to the number of
-times each arc in the program is executed. The counts are cumulative;
-each time the program is executed, it attempts to combine the existing
-@code{.da} files with the new counts for this invocation of the
-program. It ignores the contents of any @code{.da} files whose number of
-arcs doesn't correspond to the current program, and merely overwrites
-them instead.
-
-All three of these files use the functions in @code{gcov-io.h} to store
-integers; the functions in this header provide a machine-independent
-mechanism for storing and retrieving data from a stream.
+@itemize @bullet
+@item
+GCOV_PREFIX contains the prefix to add to the absolute paths
+in the object file. Prefix must be absolute as well, otherwise its
+value is ignored. The default is no prefix.
+
+@item
+GCOV_PREFIX_STRIP indicates the how many initial directory names to strip off
+the hardwired absolute paths. Default value is 0.
+
+@emph{Note:} GCOV_PREFIX_STRIP has no effect if GCOV_PREFIX is undefined, empty
+or non-absolute.
+@end itemize
+For example, if the object file @file{/user/build/foo.o} was built with
+@option{-fprofile-arcs}, the final executable will try to create the data file
+@file{/user/build/foo.gcda} when running on the target system. This will
+fail if the corresponding directory does not exist and it is unable to create
+it. This can be overcome by, for example, setting the environment as
+@samp{GCOV_PREFIX=/target/run} and @samp{GCOV_PREFIX_STRIP=1}. Such a
+setting will name the data file @file{/target/run/build/foo.gcda}.
+
+You must move the data files to the expected directory tree in order to
+use them for profile directed optimizations (@option{--use-profile}), or to
+use the @command{gcov} tool.
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